Computer systems commonly have arrangements where certain registers are used in the system for controlling the mode of the system in one way or another. These registers, sometimes called flag registers, accept a bit and, based upon that bit, reconfigure the system for a particular type of operation. One type of such control bit configuration is shown in the co-pending patent application which is being filed concurrently herewith and entitled "Processing System Using Dynamic Selection of Big Endian and Little Endian Coding," Ser. No. 387,325, which application is incorporated herein by reference. Sometimes these bits control row size or column size of a memory and sometimes these bits control other functions, some of which can be critical to the proper operation of the system.
Because of the critical nature of such flag registers, it is imperative that once the bits of the flag register are set, they not be changed under any condition until a new initialization of the system. However, as systems presently stand, mistakes in programming or mistakes in presentation of data on a bus could cause a register to change and thus reconfigure the system with drastic results.
In the prior art systems, a user is instructed not to change the bits in the flag register during normal operation. However, this requires extreme care. Sometimes safeguards are built into the software. However, in most systems an experienced operator can bypass the software safeguards. In addition, mistakes in the software in one form or another, accidentally or on purpose, as for example, by a virus, could cause the system to change, again with drastic negative results.
Thus, there is a need in the art for a system which will allow certain registers to contain critical control information and which will allow these registers to be changed by a user. However, once the user determines that the information within the register should no longer be changeable, the register must be locked closed so that even mistakes in programming will not affect the register.